Process for isomerization of alkylaromatics

ABSTRACT

An improved process for the isomerization of non-equilibrium C 8  aromatics is presented which utilizes a catalytic composition prepared by a novel method of incorporating magnesium into a crystalline aluminosilicate. The catalyst comprises an alumina matrix, a magnesium-containing zeolite, and a Group VIII metal component. It has also been found that the method of magnesium addition can dramatically affect the selectivity to para-xylene, as measured by the loss of C 8  aromatics due to undesirable side-reactions. The method of the instant invention involves addition of the magnesium to a hydrogel comprising pseudoboehmite and zeolite.

FIELD OF THE INVENTION

This invention relates to an improved process for the isomerization ofxylenes and conversion of ethylbenzene. More specifically, the inventionutilizes a catalyst composition consisting essentially of alumina, aGroup VIII metal component, and a magnesium form zeolite.

BACKGROUND OF THE INVENTION

The xylenes, namely ortho-xylene, meta-xylene and para-xylene, areimportant chemicals and find wide and varied application in industry.Ortho-xylene is a reactant for the production of phthalic anhydride.Meta-xylene is used in the manufacture of plasticizers, azo dyes, woodpreservers, etc. Para-xylene upon oxidation yields terephthalic acidwhich is used in the manufacture of synthetic textile fibers.

As a result of the important applications to which the individual xyleneisomers are subjected, it is often very important to be able to producehigh concentrations of a particular xylene. This can be accomplished byconverting a non-equilibrium mixture of the xylene isomers, whichmixture is low in the desired xylene isomer, to a mixture whichapproaches equilibrium concentrations. Various catalysts and processeshave been devised to accomplish the isomerization process. For example,it is well known in the art that catalysts such as aluminum chloride,boron fluoride, liquid hydrofluoric acid, and mixtures of hydrofluoricacid and boron fluoride can be used to isomerize xylene mixtures.

Industrially, isomerization of xylenes and conversion of ethylbenzene isperformed to produce para-xylene. A typical processing scheme for thisobjective comprises: (a) isomerizing a C₈ alkylaromatic mixture to nearequilibrium in an isomerization reaction zone; (b) separating outpara-xylene using, for example, molecular sieve technology, to obtain apara-xylene rich stream and a stream rich in other xylenes; and, (c)recycling the stream rich in other xylenes to the isomerization reactionzone.

The present invention is particularly concerned with the isomerizationreaction step which may be used in an overall process directed topara-xylene production. An important parameter to consider in thisisomerization reaction step is the degree of approach to xyleneequilibrium achieved. The approach to equilibrium that is used is anoptimized compromise between high C₈ aromatic ring loss at highconversion (i.e. very close approach to equilibrium) and high utilitycosts due to the large recycle rate of unconverted ethylbenzene,orthoxylene, and meta-xylene. Also contributing to the recycle streamare C₈ naphthenes which result from the hydrogenation of ethylbenzene.

It is desirable to run the isomerization process as close to equilibriumas possible in order to maximize the para-xylene yield, however,associated with this is a greater cyclic C₈ loss due to sidereactions.Cyclic C₈ hydrocarbons include xylenes, ethylbenzene, and C₈ naphthenes.The correlation of cyclic C₈ loss versus the distance from xyleneequilibrium is a measure of catalyst selectivity. Thus there is a strongincentive to develop a catalyst formulation which minimizes cyclic C₈loss while maximizing para-xylene yield.

Numerous catalysts have been proposed for use in xylene isomerizationprocesses such as mentioned above. More recently, a number of patentshave disclosed the use of crystalline aluminosilicate zeolite-containingcatalysts for isomerization and conversion of C₈ alkylaromatics.Crystalline aluminosilicates generally referred to as zeolites, may berepresented by the empirical formula:

    M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O

in which n is the valence of M which is generally an element of Group Ior II, in particular, sodium, potassium, magnesium, calcium, strontium,or barium, and x is generally equal to or greater than 2. Zeolites haveskeletal structures which are made up of three-dimensional networks ofSiO₄ and AlO₄ tetrahedra, corner-linked to each other by shared oxygenatoms. Zeolites with high SiO₄ /Al₂ O₃ ratio have received muchattention as components for isomerization catalysts. Representative ofzeolites having such high proportion of SiO₄ include mordenite and theZSM variety. In addition to the zeolite component, certain metalpromoters and inorganic oxide matrices have been included inisomerization catalyst formulations. Examples of inorganic oxidesinclude silica, alumina, and mixtures thereof. Metal promoters such asGroup VIII or Group III metals of the Periodic Table, have been used toprovide a dehydrogenation functionality. The acidic function can besupplied by the inorganic oxide matrix, the zeolite, or both.

When employing catalysts containing zeolites for the isomerization ofalkylaromatics, characteristics such as acid site strength, zeolite porediameter, and zeolite surface area become important parameters toconsider during formulation development. Variation of thesecharacteristics in a way that reduces side-reactions, such as,transalkylation, is required in order to achieve acceptable levels ofcyclic C₈ loss.

It has been found that, if a catalyst is formulated with the components,and in the manner set forth hereinafter, an improved process for theconversion of a non-equilibrium mixture of xylenes containingethylbenzene is obtained.

OBJECTS AND EMBODIMENTS

A principal object of the present invention is to provide an improvedhydrocarbon conversion process and a novel catalyst composition forsame. More specifically, the instant invention is aimed at an improvedprocess for the isomerization of alkylaromatic hydrocarbons with minimalloss of C₈ aromatic hydrocarbons. Accordingly, a broad embodiment of theinvention is directed toward a process for the isomerization of anon-equilibrium feed mixture of xylenes containing ethylbenzenecomprising contacting the feed mixture in the presence of hydrogen atisomerization process conditions with a catalyst consisting essentiallyof an alumina matrix, at least one Group VIII metal component, and 1 to50 wt. % of a magnesium-containing zeolite, wherein the zeolite iseither mordenite or a pentasil.

Another embodiment is directed toward a process for the isomerization ofa feed stream comprising a non-equilibrium mixture of xylenes containingethylbenzene, which comprises contacting the feed in the presence ofhydrogen at a temperature of from about 300® to 500° C., a pressure offrom about 5 to about 15 atmospheres, a liquid hourly space velocity offrom about 0.5 to about 10 hr⁻¹ with a catalyst consisting essentiallyof 75 to 95 wt. % gamma-alumina, 0.1 to 5 wt. % platinum, and 5 to 25wt. % magnesium-containing mordenite wherein said catalyst is preparedby: (a) contacting a hydrogel comprising pseudo-boehmite and mordenitewith an aqueous magnesium solution at a temperature of from 25° to 100°C. for 1 to 24 hours; (b) drying and calcining the resultant hydrogel ofstep (a) to convert the pseudo-boehmite alumina to essentiallygamma-alumina; and, (c) impregnating the calcined hydrogel of step (b)with platinum.

INFORMATION DISCLOSURE

The prior art recognizes numerous isomerization processes employing avariety of catalyst formulations. However, it is believed that none ofthe prior art processes recognizes the use of the catalyst formulationand method of making same which forms an integral part of the instantinvention.

U.S. Pat. No. 3,792,100 (Sonoda et al) teaches a process for isomerizingxylenes using a catalyst composition comprising mordenite, which hassupported thereon at least one metal selected from the group consistingof copper, silver, and chromium. This reference specifically teaches theremoval of alkali or alkaline earth metal ions from the mordenite toallow for the addition of the above-named metals. No reference is madeto the utility of either magnesium or a Group VIII metal.

In another reference, U.S. Pat. No. 3,912,659 (Brandenburg et al), acatalyst composite useful for conversion of alkylaromatic hydrocarbons,is disclosed. Specifically, the catalyst is used in a disproportionationprocess, such as, conversion of toluene into benzene and mixed xylenes.The catalyst comprises a hydrogen form mordenite, an eta or gammaalumina binder, and a sulfided Group VIII metal impregnated on saidmordenite. Patentee fails to disclose the utility of amagnesium-containing mordenite.

U.S. Pat. No. 4,159,282 (Olson et al) teaches a process forisomerization of C₈ alkylaromatics using a catalyst preferablycontaining a pentasil zeolite. Reference is made to the possiblemodification of the zeolite by incorporating therewith an amount of adifficultly reducible oxide, such as, magnesium. However, this referenceis silent to the unique combination of a Group VIII metal, magnesium,zeolite, and gamma-alumina as disclosed herein.

A reference similar to the '282 patent is U.S. Pat. No. 4,482,773 (Chuet al) which discloses an isomerization process wherein C₈ aromatics areprocessed over a catalyst comprising HZSM-5, platinum and a Group II-Ametal. Magnesium is the preferred Group II-A metal. However, the patentdoes not teach the use of either mordenite or an alumina matrix.

U.S. Pat. No. 4,100,262 (Pelrine) discloses a method of preparation ofzeolite ZSM-5 wherein in one embodiment, the patentee teaches that theoriginal cations of the as synthesized ZSM-5 can be replaced withhydrogen, rare earth metals, aluminum, metals of Groups IIA, IIIB, IVB,VIB, VIII, IB, IIB, IIIA, and IVA. Similarly, U.S. Pat. No. 4,218,573(Tabak et al) discloses a process for isomerizing xylenes wherein thezeolite ZSM-5 used in the catalyst composition may be base exchangedwith cations, such as, magnesium. Neither of the two patents recognizedthe utility of mordenite nor do the references disclose the novel methodof preparation of the instant invention.

In summary, it appears that the prior art only generally recognizes thatzeolites have utility for isomerization of C₈ alkylaromatics and that nosingle reference teaches nor suggests the invention claimed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents the loss of C₈ aromatic hydrocarbons experienced atdifferent levels of paraxylene production of Catalysts A and B of theinvention and Catalyst C, which is a magnesium deficient catalyst.

FIG. 2 represents the performance of a Catalyst E of the invention buthaving a higher magnesium content and a control Catalyst D havingmagnesium added by a different procedure.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, this invention is concerned with the catalyticisomerization and conversion of a non-equilibrium mixture of C₈ aromatichydrocarbons utilizing a catalyst consisting essentially of alumina, atleast one Group VIII metal component, and 1 to 50 wt. % of a magnesiumzeolite, wherein the zeolite is either mordenite or a pentasil. Theimproved process of the instant invention allows for a closer approachto xylene equilibrium resulting in a greater yield of para-xylenewithout the high loss of C₈ aromatics common to prior art processes.

The process of this invention is applicable to the isomerization ofisomerizable alkylaromatic hydrocarbons of the general formula:

    C.sub.6 H.sub.(6-n) R.sub.n

where n is an integer from 2 to 5 and R is CH₃, C₂ H₅, C₃ H₇, or C₄ H₉,in any combination and including all the isomers thereof. Suitablealkylaromatic hydrocarbons include, for example, ortho-xylene,metaxylene, para-xylene, ethylbenzene, ethyltoluenes, thetrimethylbenzenes, the diethylbenzenes, the triethylbenzenes,methylpropylbenzenes, ethylpropylbenzenes, the diisopropylbenzenes, thetriisopropylbenzenes, etc., and mixtures thereof.

It is contemplated that any aromatic C₈ mixture containing ethylbenzeneand xylene may be used as feed to the process of this invention.Generally, such mixture will have an ethylbenzene content in theapproximate range of 5 to 50 wt. %, an ortho-xylene content in theapproximate range of 0 to 35 wt. %, a meta-xylene content in theapproximate range of 20 to 95 wt. % and a para-xylene content in theapproximate range of 0 to 15 wt. %. It is preferred that theaforemention C₈ aromatics comprise a non-equilibrium mixture. The feedto the instant process, in addition to C₈ aromatics, may containnonaromatic hydrocarbons, i.e. naphthenes and paraffins in an amount upto 30 wt. %.

The alkylaromatic hydrocarbons for isomerization may be utilized asfound in selective fractions from various refinery petroleum streams,e.g., as individual components or as certain boiling range fractionsobtained by the selective fractionation and distillation ofcatalytically cracked gas oil. The process of this invention may beutilized for conversion of isomerizable aromatic hydrocarbons when theyare present in minor quantities in various streams. The isomerizablearomatic hydrocarbons which may be used in the process of this inventionneed not be concentrated. The process of this invention allows theisomerization of alkylaromatic containing streams such as reformate toproduce specified xylene isomers, particularly para-xylene, thusupgrading the reformate from its gasoline value to a high petrochemicalvalue.

According to the process of the present invention, an alkylaromatichydrocarbon charge stock, preferably in admixture with hydrogen, iscontacted with a catalyst of the type hereinafter described in analkylaromatic hydrocarbon isomerization zone. Contacting may be effectedusing the catalyst in a fixed bed system, a moving bed system, afluidized bed system, or in a batch-type operation. In view of thedanger of attrition loss of the valuable catalyst and of operationaladvantages, it is preferred to use a fixed bed system. In this system, ahydrogen-rich gas and the charge stock are preheated by suitable heatingmeans to the desired reaction temperature and then passed into anisomerization zone containing a fixed bed of catalyst. The conversionzone may be one or more separate reactors with suitable meanstherebetween to ensure that the desired isomerization temperature ismaintained at the entrance to each zone. It is to be noted that thereactants may be contacted with the catalyst bed in either upward,downward, or radial flow fashion, and that the reactants may be in theliquid phase, a mixed liquid-vapor phase, or a vapor phase whencontacted with the catalyst.

The process of this invention for isomerizing an isomerizablealkylaromatic hydrocarbon is preferably effected by contacting thealkylaromatic, in a reaction zone containing an isomerization catalystas hereinafter described, with a fixed catalyst bed by passing thehydrocarbon in a down-flow or radial flow fashion through the bed, whilemaintaining the zone at proper alkylaromatic isomerization conditionssuch as a temperature in the range from about 0°-600° C. or more, and apressure of atmospheric to about 100 atmospheres or more. Preferably,the operating temperature ranges from about 300°-500° C. and thepressure ranges from 0.5-55 atmospheres. The hydrocarbon is passed,preferably, in admixture with hydrogen at a hydrogen to hydrocarbon moleratio of about 0.5:1 to about 25:1 or more, and at a liquid hourlyhydrocarbon space velocity of about 0.1 to about 20 hr⁻¹ or more, mostpreferably at 0.5 to 10 hr⁻¹. Other inert diluents such as nitrogen,argon, etc., may be present.

In accordance with the present invention, the catalytic compositecomprises an alumina matrix. This matrix is a porous refractoryinorganic oxide material having the basic chemical formula of Al₂ O₃.Suitable alumina materials are the crystalline aluminas known as gamma-,eta-, and theta-alumina, with gamma- or eta-alumina giving best results.In addition, in some embodiments, the alumina carrier material maycontain minor proportions of other well known refractory inorganicoxides such as silica, zirconia, magnesia, etc.; however, the preferredsupport is substantially pure gamma- or eta-alumina. Preferred carriermaterials have an apparent bulk density of about 0.3 to about 0.8 g/ccand surface area characteristics such that the average pore diameter isabout 20 to 300 angstroms, the pore volume is about 0.1 to about 1 cc/gand the surface area is about 100 to about 500 m² /g. In general, bestresults are typically obtained with a gamma-alumina carrier materialwhich is used in the form of spherical particles having: a relativelysmall diameter (i.e. typically about 1/16-inch), an apparent bulkdensity of about 0.3 to about 0.8 g/cc, a pore volume of about 0.7 ml/g,and a surface area of about 150 to about 250 m² /g.

The preferred alumina carrier material may be prepared in any suitablemanner and may be synthetically prepared or naturally occurring.Whatever type of alumina is employed, it may be activated prior to useby one or more treatments including drying, calcination, steaming, etc.,and it may be in a form known as activited alumina, activated alumina ofcommerce, porous alumina, alumina gel, etc. For example, the aluminacarrier may be prepared by adding a suitable alkaline reagent, such asammonium hydroxide, to a salt of aluminum such as aluminum chloride,aluminum nitrate, etc., in an amount to form an aluminum hydroxide gelwhich upon drYing and calcining is converted to alumina. The aluminacarrier may be formed in any desired shape such as spheres, pills,cakes, extrudates, powders, granules, tablets, etc., and utilized in anydesired size. For the purpose of the present invention, a particularlypreferred form of alumina is the sphere, and alumina spheres may becontinuously manufactured by the well-known oil drop method whichcomprises: forming an alumina hydrosol by any of the techniques taughtin the art and preferably by reacting aluminum metal with hydrochloricacid, combining the resultant hydrosol with a suitable gelling agent anddropping the resultant mixture into an oil bath maintained at elevatedtemperatures. The droplets of the mixture remain in the oil bath untilthey set and form hydrogel spheres. The spheres are then continuouslywithdrawn from the oil bath and typically subjected to specific agingtreatments in oil and an ammoniacal solution to further improve theirphysical characteristics. The resulting aged and gelled particles arethen washed and dried at a relatively low temperature of about 50°-200°C. and subjected to a calcination procedure at a temperature of about450°-700° C. for a period of about 1 to about 20 hours. This treatmenteffects conversion of the alumina hydrogel to the correspondingcrystalline gamma-alumina. See the teachings of U.S. Pat. No. 2,620,314for additional details.

An especially preferred method of preparing the alumina matrix involvesthe inclusion of a zeolite into the alumina hydrosol prior to droppingthe hydrosol into an oil bath. This technique yields a hydrogelcomprising pseudo-boehmite alumina and zeolite. The amount of zeoliteadded to the hydrosol can range from 1 to 50 wt. % zeolite based on theweight of the finished catalyst composite. Prior to drying and calciningthe resultant hydrogen containing pseudo-boehmite alumina and zeolite,the hydrogel may be subjected to any number of steps to incorporateelements selected from the Group I-A to Group VIII metals. Calcinationof the hydrogel transforms the pseudo-boehmite alumina into the morestable and commercially usable gamma-alumina.

The zeolite component of the present invention may be selected fromeither mordenite or a pentasil. Mordenite is a crystallinealuminosilicate of the zeolite type which is well known to the art as anadsorption agent and as a catalytic agent in hydrocarbon conversionreactions. Mordenite, as typically manufactured or found in nature, ishighly siliceous and is characterized by a silica (SiO₂) to alumina (Al₂O₃) mole ratio of about 10. Two synthetic types of mordenite areavailable; "large-port" and "small-port" mordenites. Studies of theadsorption behavior of mordenite and the first synthetic types indicatedthat the pore structure was considerably smaller than the structureindicated. These types are referred to as small-port mordenite; theyexhibit an adsorption diameter of about 4 angstroms. By varyingsynthesis conditions, a large-port mordenite has been synthesized whichpossesses the adsorption properties expected for the structure. Afteractivation (dehydration), large-port mordenite adsorbs large moleculessuch as benzene and cyclohexane which are completely excluded by thesmall-port variety. The large-port mordenite is preferred in the instantinvention. The mordenite crystalline structure comprises four-andfive-membered rings of SiO₄ and AlO₄ tetrahedra so arranged that theresulting crystal lattice comprises pores and channels running parallelalong the crystal axis to give a tubular configuration. This structureis unique among some zeolite crystalline aluminosilicates in that thechannels do not intersect and access to the cages or activities can beonly one direction. For this reason, the mordenite structure isfrequently referred to as two-dimensional in contrast to the other knowncrystalline aluminosilicates such as faujasite in which the cavities canbe entered from three directions.

As stated, mordenite, as commercially available, has an SiO₂ /Al₂ O₃mole ratio of about 10 and is usually characterized as being in thesodium form. Before the sodium form of mordenite can be utilized as aneffective catalyst for hydrocarbon conversion reactions, it must befirst converted to the hydrogen form and/or ion exchanged to replace thealkali metal ion (typically sodium) with a desired metal cation.Mordenite, since it has a high initial SiO₂ /Al₂ O₃ mole ratio and ismore acid resistant than faujasite, may be converted to the hydrogenform by replacing the sodium ion with a hydrogen ion by treatment withan aqueous solution of a mineral acid. Alternatively, the hydrogen ioncan be incorporated by ion exchange with ammonium hydroxide and thencalcining the ammonium form mordenite. Hydrogen ion exchanged mordenitesare often termed H-mordenite and are illustrated in U.S. Pat. No.3,281,482. The catalytic activity of mordenites may also be increased byextracting a portion of the alumina from the mordenite crystalstructure, as well as simultaneously ion exchanging hydrogen ions, bytreatment with mineral acids under relatively severe temperatures andcontact time. What is produced are aluminum-deficient mordenitesmaintaining the same gross crystal structure in terms of grossinteratomic distances as the original mordenite, as measured by X-raydiffraction patterns. Mordenites that have been so acid extractedtypically have an SiO₂ /Al₂ O₃ ratio in excess of 25:1 which may extendto 100:1 or more. These acid extracted mordenites are exemplified byU.S. Pat. No. 3,480,539. Acid extracted mordenites which areparticularly effective and active catalysts have SiO₂ /Al₂ O₃ ratios inexcess of 50:1. However, we have found that the use of dealuminatedmordenites causes increased loss of C₈ cyclics. Although the mechanismis not proven, we speculate that the increased cyclics loss is due to anincrease in zeolite pore diameter upon dealumination. This increasedpore size reduces steric constraints on the transition state fortransalkylation, leading to increased C₈ cyclics loss bytransalkylation.

It is preferred that sodium cation removal be accomplished by ammoniumion exchange. The NH₄ -mordenite that results is converted to hydrogenform mordenite during calcination. In the preferred embodiment, thisammonium ion exchange is already achieved to some extent by thetreatment of the hydrogel spheres with ammoniacal solutions duringaging, and completed during the subsequent washing with 0.5 wt. % NH₃/H₂ O solution at 95° C.

Alternatively, the zeolite component of the present invention may be apentasil crystalline aluminosilicate zeolite. "Pentasil" is a term usedto describe a class of shape selective zeolites. This novel class ofzeolites is well known to the art and is typically characterized by asilica-to-alumina mole ratio of at least about 12. Suitable descriptionsof the pentasils may be found in U.S. Pat. Nos. 4,159,282, 4,163,018,and 4,278,565, all of which are incorporated herein by reference. Of thepentasil zeolites, the preferred ones are ZSM-5, ZSM-8, ZSM-11, ZSM-23,and ZSM-35, with ZSM-5 being particularly preferred.

It is also within the scope of the present invention that the particularpentasil selected may be a gallosilicate. Gallosilicates haveessentially the same structure as the ZSM-type zeolites describedhereinabove, except that all or part of the aluminum atoms in thealuminosilicate crystal framework are replaced by gallium atoms. Thissubstitution of the aluminum by gallium is usually performed prior to orduring synthesis of the zeolite. The gallium content for this particulartype of pentasil, expressed as mole ratios of SiO₂ to Ga₂ O₃, range from20:1 to 400:1 or more.

Regardless of the type of zeolite used, it is desired that the zeolitebe predominantly in the sodium form prior to its commingling with thealumina matrix. In a preferred embodiment, the zeolite is commerciallyobtained in the sodium form and used directly with the alumina hydrosol.The hydrosol is then dispersed into an oil bath as described above toform a hydrogel.

Upon forming the hydrogel containing the pseudo-boehmite alumina andzeolite, the addition of magnesium to zeolite is performed. Although notexactly understood, superior results are obtained, as exemplified in theexamples to follow, when the magnesium is introduced to the zeolite atthe hydrogel stage as opposed to either the addition after the hydrogelhas been dried and calcined or to the zeolite prior to addition to thealumina hydrosol. It is believed that the zeolite in the hydrogel ismore readily susceptible to ion exchange with magnesium. It is alsobelieved that having pseudo-boehmite present, as opposed togamma-alumina, during contacting with the magnesium solution greatlyreduces the possibility that the magnesium will ion exchange with thealumina matrix. Thus, a more efficient use of magnesium is obtained byfollowing the method of the instant invention. The amount ofmagnesium-containing zeolite may range from 1 to 50 wt. % baseo on theweight of the finished catalyst composite. Preferably, the amount ofmagnesium-containing zeolite ranges from 5 to 25 wt. % of the finishedcatalyst.

Any suitable magnesium compound may be used to introduce the magnesiumcation into the zeolite. Representative magnesium compounds includemagnesium nitrate, magnesium benzoate, magnesium propionate, magnesium2-ethylhexoate, magnesium carbonate, magnesium formate, magnesiumoxalate, magnesium amide, magnesium bromide, magnesium chloride,magnesium acetate, magnesium lactate, magnesium laurate, magnesiumoleate, magnesium palmitate, magnesium silicylate, magnesium stearate,and magnesium sulfide. It is preferred that the magnesium compound be insolution in order to facilitate the contact with the hydrogel. Anysolvent relatively inert to the hydrogel and the magnesium compound maybe employed. Suitable solvents include water and aliphatic, aromatic, oralcoholic liquid.

Contacting the magnesium-containing solution with the hydrogel iscarried out at a temperature from 0° to 150° C. The preferredtemperature range is from about 25° to 100° C. The time of contact canvary from about 1 to 24 hours. The physical means for contacting themagnesium-containing solution with hydrogel can be accomplished by aplurality of methods, with no one method having a particular advantage.Such contacting methods may include, for example, a stationary bed ofhydrogel particles in an agitated solution, a stationary bed of hydrogelparticles in a continuously flowing solution, a stationary bed ofhydrogel particles in a static solution or any other means whichefficiently contacts the magnesium-containing solution with the hydrogelcomprising the pseudo-boehmite and zeolite.

The amount of magnesium incorporated into the zeolite should be suchthat greater than 50% of the available ion exchange sites are occupied.The amount of magnesium may range from 0.1 wt. % of the finishedcatalyst to as high as 10 wt. %. By "finished catalyst", it is meant thefinal catalyst formulation suitable for contact with the hydrocarbonfeed. Preferably, the amount of magnesium present in the finishedcatalyst composition is between 0.5 and 5 wt. %.

It is contemplated that other metals may be directly substituted inplace of magnesium to provide the desired feature of the invention,namely, allowing for a close approach to xylene equilibrium whileminimizing the loss of C₈ aromatics. Such alternative metals includecalcium, lanthanum, and copper or mixtures of these metals.

The catalyst of the instant invention also contains at least one GroupVIII metal component. Preferably, this Group VIII metal is selected fromthe platinum group metals. Of the platinum group metals, which includepalladium, rhodium, ruthenium, osmium and iridium, the use of platinumis preferred. The platinum group component may exist within the finalcatalyst composite as a compound such as an oxide, sulfide, halide,oxysulfide, etc., or as an elemental metal or in combination with one ormore other ingredients of the catalyst. It is believed that the bestresults are obtained when substantially all the platinum group componentexists in the elemental state. The platinum group component generallycomprises from about 0.01 to about 2 wt. % of the final catalyticcomposite, calculated on an elemental basis. It is preferred that theplatinum content of the catalyst be between about 0.1 and 1 wt. %. Thepreferred platinum group component is platinum, with palladium being thenext preferred metal. The platinum group component may be incorporatedinto the catalyst composite in any suitable manner such as bycoprecipitation or cogelation with the preferred carrier material, or byion-exchange or impregnation of the carrier material. The preferredmethod of preparing the catalyst normally involves the utilization of awater-soluble, decomposable compound of a platinum group metal toimpregnate the calcined hydrogel material. For example, the platinumgroup component may be added to the calcined hydrogel by commingling thecalcined hydrogel with an aqueous solution of chloroplatinic orchloropalladic acid. An acid such as hydrogen chloride is generallyadded to the impregnation solution to aid in the distribution of theplatinum group component through the calcined hydrogel particles.

After addition of the Group VIII metal component, the calcined hydrogelcomprising gamma-alumina, magnesium-containing zeolite, and platinum isdried at a temperature ranging from about 100° to about 320° C. for aperiod of at least 2 to about 24 hours or more, and finally calcined oroxidized at a temperature ranging from about 450° to about 650° C. inair or oxygen atmosphere for a period of about 0.5 to about 10 hours inorder to convert all of the metallic components to the correspondingoxide form. The resultant oxidative composite is preferably subjected toa substantially water-free reduction step prior to its use in theisomerization of hydrocarbons. This step is designed to selectivelyreduce the platinum group component to the elemental metallic state,while maintaining the magnesium component in a positive oxidation state,and to ensure a uniform and finely divided dispersion of the metalliccomponents throughout the catalyst. Preferably, a substantially pure anddry hydrogen stream (i.e. less than 20 vol. ppm H₂ O) is used as thereducing agent in this step. The reducing agent is contacted with theoxidized catalyst at conditions including a reduction temperatureranging from about 200° to about 650° C. and a period of time of about0.5 to 10 hours effective to reduce substantially all of the platinumgroup component to the elemental metallic state.

The resulting reduced catalytic composite may, in some cases, bebeneficially subjected to a presulfiding operation designed toincorporate in the catalytic composite from about 0.05 to about 0.5 wt.% sulfur calculated on an elemental basis. Preferably, this presulfidingtreatment takes place in the presence of hydrogen and a suitablesulfurcontaining compound such as hydrogen sulfide, lower molecularweight mercaptans, organic sulfides, etc. Typically, this procedurecomprises treating the reduced catalyst with a sulfiding gas such as amixture of hydrogen and hydrogen sulfide having about 10 moles ofhydrogen per mole of hydrogen sulfide at conditions sufficient to effectthe desired incorporation of sulfur, generally including a temperatureranging from about 10° up to about 593° C. or more. It is generally agood practice to perform this presulfiding step operation undersubstantially water-free conditions.

The following example is presented for purpose of illustration only andis not intended to limit the scope of the present invention.

EXAMPLE

This example presents the results from four different processes. Eachprocess was evaluated using a pilot plant flow reactor processing anon-equilibrium C₈ aromatic feed comprising 52.2 wt. % metaxylene, 18.7wt. % ortho-xylene, 0.1 wt. % para-xylene, 21.3 wt. % ethylbenzene, and0.1 wt. % toluene, with the balance being nonaromatic hydrocarbons. Thisfeed was contacted with 100 cc of catalyst at a liquid hourly spacevelocity of 2, and a hydrogen to hydrocarbon mole ratio of 4. Reactorpressure and temperature were adjusted to cover a range of conversionvalues in order to develop the relationship between C₈ ring loss andapproach to xylene equilibrium (as determined by product para-xylene tototal xylene weight ratio). At the same time, at each temperature, thepressure was chosen to maintain a constant mole ratio of C₈ naphthenesto C₈ aromatics of approximately 0.06.

Initial catalyst preparation for each of the processes describedhereinbelow proceeded as follows. A first solution was prepared byadding a zeolite to enough alumina hydrosol, prepared by digestingmetallic aluminum in hydrochloric acid, to yield a zeolite content inthe finished catalyst equal to about 10 wt. %. As described hereinbelow,the zeolite was either a mordenite or a pentasil. To this first solutionis added a second solution of hexamethylenetetramine (HMT). These twosolutions were mixed to form a homogeneous admixture which was thendispersed as droplets into an oil bath at a temperature of about 95° C.The droplets remained in the oil bath until they set and formed hydrogelspheres. The spheres were removed from the oil bath and washed with anaqueous solution containing about 0.5 wt. % ammonia. At this point inthe preparation the hydrogel spheres which are commonly referred to as"wet hopper spheres" (WHS), were either directly dried and calcined orcontacted with a magnesium-containing solution.

The first process, designated as Run A, which is in accordance with theinvention, utilized a catalyst wherein the zeolite contained in thehydrogel was hydrogen/ammonium form mordenite. A stationary bed of WHS,comprising the mordenite and pseudo-boehmite alumina, was contacted withan aqueous solution of 1.5 molal magnesium acetate by continuouslycirculating the solution at a rate of about 8 ml/minute. The temperatureof the magnesium acetate solution was maintained at about 94° C. for aperiod of about 20 hours. A deionized water wash using about 10 bedvolumes was performed at the conclusion of the magnesium addition step.The WHS were then air dried at 11° C. for about 12 hours and thencalcined in air at a temperature of about 650° C.

The calcined WHS were then impregnated with a solution of chloroplatinicacid, containing 2 wt. % hydrochloric acid (based on calcined WHS), toyield a final platinum content of 0.28 wt. %. The impregnated sphereswere oxidized and chloride adjusted at 525° C., reduced in anenvironment of H₂ at 565° C., and sulfided with H₂ S. The amount ofmagnesium-containing zeolite was about 9.8 wt. %, the magnesium contentwas 1.02 wt. %, and the sulfur content was 0.12 wt. %. The isomerizationperformance results from Run A are presented in FIG. 1.

Run B was also performed in accordance with the instant invention. Theprocess of Run B was essentially identical to that of Run A except thezeolite added to the alumina hydrosol comprised a hydrogen form ZSM-5zeolite having a silica to alumina mole ratio of 42. The platinum andmagnesium contents of this catalyst were 0.32 wt. % and 0.95 wt. %,respectively. The sulfur content was targeted to be 0.11 wt. %. Theisomerization performance results for Run B are also presented in FIG.1.

To demonstrate the superior performance of the process of the instantinvention, two control processes were run. The first process, designatedRun C and not in accordance with the instant invention, utilized acatalyst formulation that was prepared identically to the catalysts inRuns A and B, however, in Run C, there was no addition of magnesium tothe zeolite. The zeolite used in Run C was the same sodium formmordenite used in Run A. The finished catalyst was analyzed for bothplatinum and magnesium. The platinum content was 0.32 wt. %, themagnesium content was less than 0.05 wt. %, and the sulfur content was0.08 wt. %. Test results comparing the isomerization performance of RunC with Runs A and B are presented in FIG. 1.

The second control process, designated as Run D, also not in accordancewith the invention, was performed to demonstrate the importance of themeans by which magnesium is added to the zeolite. The catalyst used inRun D was prepared in an identical manner as the catalyst of Run Aexcept the magnesium acetate was impregnated onto the catalyst at 25° C.after the WHS had been dried and calcined but prior to platinumaddition. Thus the magnesium-containing solution was contacted withcalcined spheres comprising gamma-alumina and mordenite. The platinumand magnesium levels were 0.30 wt. % and 0.57 wt. %, respectively. Thesulfur content was 0.09 wt. %. In order to accurately access theperformance of this second control process, in particular, in order toconduct a comparison at the same Mg levels, it was necessary to prepareanother catalyst in accordance with the instant invention following theprocedure used for the catalyst of Run A. In preparing this catalyst, atemperature of 25° C. was maintained during the contact of the WHS withthe magnesium acetate solution. The platinum content was 0.3 wt. %, themagnesium content was 0.56 wt. %, and the sulfur content was 0.02 wt. %.This catalyst was tested as Run E. A comparison of results for Run D,not of the invention, and Run E, in accordance with the invention, isgraphically depicted in FIG. 2.

Both FIGS. 1 and 2 graphically illustrate the same performanceparameters. The x-axis is the concentration of para-xylene in theproduct, expressed as mole % relative to the total xylenes in theproduct. The y-axis represents the amount of C₈ cyclic hydrocarbons lostdue to side reactions. This parameter is defined as the sum of C₈aromatics and naphthenes in the feed minus the amount of C₈ aromaticsand naphthenes in the product divided by the C₈ aromatic and naphthenesin the feed.

The isomerization performance results presented in both FIGS. 1 and 2clearly indicate the advantage of the process of the instant invention.More specifically, if the results of the process of the instantinvention is compared to those of the control runs, while operating atconditions to produce a product containing 22 mole percent para-xylene,about 25% less C₈ aromatic hydrocarbons are lost due to side reactions.

What is claimed is:
 1. A process for the isomerization of anon-equilibrium feed mixture of xylenes containing ethylbenzenecomprising contacting the feed mixture in the presence of hydrogen atisomerization process conditions with a catalyst containing 0.2-2.0 wt.% magnesium and consisting essentially of an alumina matrix, at leastone Group VIII metal component, and 1 to 50 wt. % of amagnesium-containing zeolite, wherein the magnesium containing zeoliteis prepared by contacting a hydrogel comprising pseudo-boehmite aluminaand zeolite with an aqueous magnesium solution.
 2. The process of claim1 wherein the isomerization process conditions comprise a temperature offrom about 300° C. to about 500° C., a pressure of from about 0.5 toabout 55 atmospheres, a liquid hourly space velocity of from about 0.5to 10 liquid volumes of the nonequilibrium mixture of xylenes containingethylbenzene per hour per volume of catalyst.
 3. The process of claim 1wherein the catalyst is spherical in shape and contains between 50 and99 wt. % gamma-alumina.
 4. The process of claim 1 wherein the catalystcontains between 0.1 and 5 wt. % platinum.
 5. The process of claim 1wherein the zeolite is mordenite.
 6. The process of claim 1 wherein thezeolite is a pentasil selected from the group consisting of ZSM-5,ZSM-8, ZSM-11, ZSM-12, ZSM-23, and ZSM-35.
 7. The process of claim 6wherein the pentasil is a gallosilicate.
 8. The process of claim 1wherein the aqueous magnesium solution is contacted with the hydrogel ata temperature ranging from 25° to 100° C. for 1 to 24 hours.
 9. Theprocess of claim 8 wherein after contacting with the aqueous magnesiumsolution, the hydrogel is dried at a temperature of from 50° to 200° C.and calcined to convert the pseudo-boehmite alumina to essentiallygamma-alumina prior to addition of the Group VIII metal component. 10.The process of claim 9 wherein the hydrogel is prepared by the oil dropmethod.
 11. A process for the isomerization of a feed stream comprisinga non-equilibrium mixture of xylenes containing ethylbenzene, whichcomprises contacting the feed in the presence of hydrogen at atemperature of from about 300° to 500° C., a pressure of from about 0.5to about 55 atmospheres, a liquid hourly space velocity of from about0.5 to about 10 hr⁻¹ with a catalyst containing 0.2-2.0 wt. % magnesiumand consisting essentially of 75 to 95 wt. % gamma-alumina, 0.1 to 5 wt.% platinum, and 5 to 25 wt. % magnesium-containing mordenite, whereinsaid catalyst is prepared by:(a) contacting a hydrogel prepared by theoil drop method comprising pseudo-boehmite and mordenite with an aqueousmagnesium solution at a temperature of from 25° to 100° C. for 1 to 24hours; (b) drying and calcining the resultant hydrogel of step (a) toconvert the pseudo-boehmite alumina to essentially gamma-alumina; and,(c) impregnating the calcined hydrogel of step (b) with platinum.